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We present comprehensive quantitative analysis of Raman spectra in two-(Si/SiGe superlattices) and three-(Si/SiGe cluster multilayers) dimensional nanostructures. We find that the Raman spectra baseline is due to the sample surface imperfection and instrumental response associated with the stray light. The Raman signal intensity is analyzed, and Ge composition is calculated and compared with the experimental data. The local sample temperature and thermal conductivity are calculated, and the spectrum of longitudinal acoustic phonons is explained.

We report the degradation of low temperature photoluminescence (PL) from Si/SiGe three-dimensional cluster morphology nanostructures under continuous photoexcitation. The PL intensity initially decreases slowly for about 15 minutes, and then decreases rapidly, until only ∼ 10% of the original PL intensity remains. A complete recovery of the PL requires restoring the sample temperature to ∼ 300K. We propose that a slow accumulation of charge in SiGe clusters enhances the rate of Auger recombination and results in the observed PL degradation.

Strain engineering in composition-controlled Si-Si/Ge nanocluster multilayers with high germanium content (~ 50%) is achieved by varying thicknesses of Si/SiGe layers and studied by low temperature photoluminescence (PL) measurements. The PL spectra show reduction in strained silicon energy bandgap and a splitting presumably associated with partial removal of heavy hole-light hole degeneracy in SiGe valence band. Time-resolved PL measurements performed under different excitation wavelengths show dramatically different PL lifetimes, ranging from ~ 2 μs to 10 ns and an unusually high PL quantum efficiency. The results are explained by using the Si/SiGe interface recombination model, which is supported by ultra-high resolution transmission and analytical electron microscopy measurements.

A quantum structure based on Si/SiO2 and fabricated using standard Si technology has strong potential for applications in non-volatile and scaled dynamic memories. Among standard requirements, such as long retention time and endurance, a structure utilizing resonant tunneling offers lower bias operation and faster write/read cycle. In addition, degradation effects associated with Fowlher-Nordheim (FN) hot electron tunneling can be avoided. Superlattices of nanometer size layers of silicon and silicon dioxide were obtained by sputtering. The size of the silicon nanocrystallites (nc-Si) is fixed by the thickness of the silicon layer which limits the size dispersion. A detailed analysis of the storage of charges in the dots, as a function of the nanocrystals size, is investigated using capacitance methods. Constant voltage and constant capacitance techniques are used to monitor the discharge of the structure. Room temperature non-volatile memory with retention times as long as months is evidenced.

Two-dimensional periodic arrays of inverted pyramid holes with nanometer scale have been patterned on the surface of a (100) silicon wafer and studied for possible application in nanoscale silicon based devices. The surface patterning employed a simple microelectronic processing scheme in which the standing wave intensity pattern from two interfering 458nm laser beams was used to expose holes in a photoresist layer. Subsequent dry etching through an underlying oxide mask layer, followed by a KOH etching step yielded a highly periodic, large area array of inverted pyramids. The pyramid geometry is formed during the anisotropic KOH etch, which stops at the (111) pyramid walls. Therefore, the tips of all inverted pyramids are formed by the intersection of (111) silicon crystal planes and have identical geometry. This study focuses on the use of these features as templates for the controlled crystallization of amorphous silicon layers and also as electric field concentrating “funnels” in MOS-type structures. We will discuss a proposed device in which silicon nanocrystals will be incorporated into the concentrated electric field region at the tip of each inverted pyramid. With this structure, the charging of identical addressable nanocrystals may be possible, leading to the development of practical nanoscale silicon devices.

Short-period superlattices consisting of nanocrystalline Si wells and amorphous SiO2 barriers were analyzed using various structural (transmission electron microscopy, atomic force microscopy, and x-ray diffraction) and optical (Raman scattering and spectroscopic ellipsometry) characterization techniques. We observe parallel layers containing polycrystalline Si wells, primarily with <111> orientation, and an interesting surface morphology due to sputtering damage. Raman spectra show a redshift and broadening due to finite-size effects. The ellipsometry data can be described using the effective medium approximation (since the superlattice period is much shorter than the wavelength of the optical excitation) or a superlattice approach based on the Fresnel equations with a polycrystalline Si dielectric function.

We have investigated the sensitivity of blue (PLmax = 480 nm) and red (PLmax = 660 nm) emitting porous silicon samples to various chemical adsorbates. Steady-state and time-resolved photoluminescence measurements and FTIR spectroscopy were employed to characterize the photophysical and optical effects induced by chemical exposure. The red samples, which are hydrogen terminated, exhibit quenching and recovery of photoluminescence intensity and broadening of the Si-Hx stretch bands upon exposure to liquid methanol. This behavior is attributed to the ability of the Si-Hx specie on the surface of the PSI to interact with the solvent molecules which temporarily traps the electrons and causes PL loss and Si-Hx broadening. The blue samples, which are oxygen terminated, display similar sensitivity to methanol. This sensitivity is attributed to the solvent's ability to change the surface passivation and thereby introduce competitive radiative and nonradiative recombination channels. The origin of the blue PL is discussed.

Room-temperature photoluminescence (PL) peaking at 1.1 eV has been found in electrochemically etched mesoporous silicon annealed at 950°C. Low-temperature PL spectra clearly show a fine structure related to phonon-assisted transitions in pure crystalline silicon (c-Si) and the absence of defect-related (e.g.P-line) and impurity-related (e.g.oxygen, boron) transitions. The maximum PL external quantum efficiency (EQE) is found to be better than 0.1% with a weak temperature dependence in the region from 12K to 400K. The PL intensity is a linear function of excitation intensity up to 100 W/cm2. The PL can be suppressed by an external electric field ≥ 105 V/cm. Room temperature electroluminescence (EL) related to the c-Si band-edge is also demonstrated under an applied bias ≤ 1.2 V and with a current density ≈ 20 mA/cm2. A model is proposed in which the radiative recombination originates from recrystallized Si grains within a non-stoichiometric Si-rich silicon oxide (SRSO) matrix.

SiGe quantum devices were demonstrated by AFM oxidation and selective wet etching with features size down to 50 nm. To passivate the devices and eliminate the interface states between Si/SiO2, low temperature regrowth of epitaxial silicon over strained SiGe has been tested. The silicon regrowth on Si0.8Ge0.2 was done by rapid thermal chemical vapor deposition (RTCVD) at 700 °C using a hydrogen pre-cleaning process at 800 °C and 10 torr. SIMS analysis and photoluminescence (PL) of strained SiGe capped with epitaxial regrown silicon show a clean interface. Nano-gaps between doped SiGe filled and overgrown with epitaxial silicon show an electrical insulating property at 4.2 K.

We report a successful unification of standard lithographic approaches (top down), anisotropic etching of atomically smooth surfaces, and controlled crystallization of silicon quantum dots (bottom up) to produce silicon nanoclusters at desired locations. These results complement our previous demonstration of silicon nanocrystal uniformity in size, shape, and crystalline orientation in nanocrystalline silicon (nc-Si)/SiO2 superlattices, and could lead to practical applications of silicon nanocrystals in electronic devices. The goal of this study was to induce silicon nanocrystal nucleation at specific lateral sites in a continuous amorphous silicon (a-Si) film. Nearly all previous studies of silicon nanocrystals are based on films containing isolated nanocrystals with random lateral position and spacing. The ability to define precise two-dimensional arrays of quantum dots would allow each quantum dot to be contacted using standard photolithographic techniques, leading to practical device applications like high-density memories. In this work, a template substrate consisting of an array of pyramid-shaped holes in a (100) silicon wafer was formed using standard microfabrication techniques. The geometry of this substrate then influenced the crystallization of an a-Si/SiO2 superlattice that was deposited on it, resulting in preferential nucleation of silicon nanoclusters near the bottom of the pyramid holes. These clusters are clearly visible in transmission electron microscopy (TEM) images, while no clusters have been observed on the planar surface areas of the template. Possible explanations for this selective nucleation and future device structures will be discussed.

Thin layers made of densely packed silicon nanocrystals sandwiched between amorphous silicon dioxide layers have been manufactured and characterized. An amorphous silicon/amorphous silicon dioxide superlattice is first grown by CVD or RF sputtering. The a-Si layers are recrystallized in a two-step procedure (nucleation + growth) to form layers of nearly identical nanocrystals whose diameter is given by the initial a-Si layer thickness. The recrystallization is monitored using a variety of techniques, including TEM, X-Ray, Raman, and luminescence spectroscopies. When the a-Si layer thickness decreases (from 25 nm to 2.5 nm) or the a-SiO2 layer thickness increases (from 1.5 nm to 6 nm), the recrystallization temperature increases dramatically compared to that of a single a-Si film. The removal of the a-Si tissue present between the nanocrystals, the passivation of the nanocrystals, and their doping are discussed.

Samples of Ge Nanowires (Ge NWs) grown by chemical vapor deposition (CVD) on single crystal, (100) and (111) oriented Si substrates were studied with respect to their electrical properties. Using different contact geometries, direct current (DC) and alternating current (AC) electrical and photoelectrical measurements were carried out at room temperature. A rectifying junction behavior was observed indicating a low defect density at NWs/substrate heterointerface. AC conductance exhibits significant frequency dependence with a power law behavior, suggesting that carrier transport in Ge NW volume is associated with hopping processes.

Three-dimensional SiGe nanostructures grown on Si using molecular beam epitaxy exhibit photoluminescence (PL) in the important spectral range of 1.3–1.6 μm. At a higher level of photo-excitation, thermal quenching of the PL intensity is suppressed and the previously accepted type II energy band alignment at Si/SiGe cluster hetero-interfaces no longer controls radiative carrier recombination. Instead, a dynamic type I energy band alignment governs the strong decrease in carrier radiative lifetime and further increase in the luminescence quantum efficiency. In contrast to the strongly temperature dependent and slow radiative carrier recombination found in bulk Si, Auger mediated PL emanating from the nanometer-thick Si layers is found to be nearly temperature independent with a radiative lifetime approaching 10−8 s, which is comparable to that found in direct band gap III-V semiconductors. Such nanostructures are thus potentially useful as CMOS compatible light emitters and in optical interconnects.

Light emission in silicon has been intensively investigated since the 1950s when crystalline silicon (c-Si) was recognized as the dominant material in microelectronics. Silicon is an indirect-bandgap semiconductor and momentum conservation requires phonon assistance in radiative electron-hole recombination (Figure 1a, top left). Because phonons carry a momentum and an energy, the typical signature of phonon-assisted recombination is several peaks in the photoluminescence (PL) spectra at low temperature. These PL peaks are called “phonon replicas.” High-purity c-Si PL is caused by free-exciton self-annihilation with the exciton binding energy of ~11 meV. The TO-phonon contribution in conservation processes is most significant, and the main PL peak (~1.1 eV) is shifted from the bandgap value (~1.17 eV) by ~70 meV—that is, the exciton binding energy plus TO-phonon energy (Figure 1a).

The enormous progress of communication technologies in the last years has increased the demand for efficient and low-cost optoelectronic functions. For several present and future applications, photonic materials—in which light can be generated, guided, modulated, amplified, and detected—need to be integrated with standard electronic circuits in order to combine the information-processing capabilities of electronics data transfer and the speed of light. Long-distance communications, local-area-networks data transfer, and chip-to-chip or even intrachip optical communications all require the development of efficient optical functions and their integration with state-of-the-art electronic functions. Silicon is the material of choice for reliable and low-cost optoelectronic integrated circuits because it is the leading semiconductor in the electronic arena and since a wellestablished processing technology exists for this material. However Si is characterized by an indirect bandgap and by a weak electro-optic effect. It is therefore not suitable for the implementation of fundamental optical functions such as light emission and modulation. At the moment, hybrid integration of compound-semiconductor optical functions with Si electronic functions is providing the gateway from electronic to photonic technology. However several strategies are being considered to engineer the optical functions of Si and to realize fully Si-based or at least Si-compatible optoelectronics.

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